Note 2 — Chapter II · Heat.
Reading edition
The combustible nature of many minerals under the surface of the earth,—which was all the realm of Hades (invisible),—would assist the notion of a fiery abode for the infernal gods. Our phrase ‘plutonic rock’ would then have a very prosaic sense. Pliny says that in his time sulphur was used to keep off evil spirits, and it is not impossible that it first came to be used as a medicine by this route.2